As other hobby rocket engines, these work constantly, and are uniform from box to box. When making sugar fuel gets tiring, or you want some quick fun, these are perfect. I got these the C6-7 ones as they were $1 less at the time, I used two the second they arrived.
The 7 was a bit more than desired (the first rocket normally works best with a C6-5) as the first rocket was fairly heavy and thus the streamer deployed late. Two less seconds would have made a fair difference between it landing gracefully versus with a thump, but once again I expected this.
The second rocket got much higher, and thus the delay was actually very nice as it helped keep the rocket within the launch area.
Ideally this would also go well on a three stage rocket as once again allowing the rocket to fall relatively straight down before the streamer or parachute starts allowing it to drift more can really help in small fields (or for those who would rather not run like crazy after the rocket in hopes of catching it.)